Surface testing device



Feb. 12, 1946. Y A, F. TURNER l 2,394,951.

SURFACE TESTING DEVICE Filed June 19, 1943 A ARTHUR FTURNER Y- .|NVENT R forming the nlm carried by the Patented Feb. 12, 1946 UNITED STATES linfluir-:rrr oFFlcE Arthur F. Turner; Brighton,- N. Y., assignor to Bausch & Lomb Optical Company, Rochester, N. Y.. a corporation of New York Application Jupe 19, 1943, serial N. 491,511

(Cl. 88'14) v member I having an optically correct reference 1 claim.

This invention relates .to testngdevices and more particularly to an optical test glass for testing highly polished surfaces.

Optical test glasses heretoforel proposed have generally comprised a transparent member having a reference surface which was brought into contact with the surface to be tested. `*The surlface under test could be compared with the reference surface by observing the interferenceI bands formed by theair film intermediate the surface being tested and the 'reference surface of the test glass. Y

Such test glasses have long been used/in the optical arts for testing the surfaces of precision lenses, and related optical elements but their use has not been great in other arts. This has" beenlargely due to the fact that when a polished metal surface is tested in the usual manner by means .of a test glass, the interference bands lack contrast. As there is very little contrast between the bands, it is difficult for an operator to see surface carrying a thin semi-transparent lm I I. The lm II may be formed on the reference lsurface of the body `I0 by any means desired, but

inthe present pref/erred form of the invention,

, the lm is\formed by a high vacuum thermal or count the number of bands. The lack of contrast is due to the very unequal reflectance of the glass surface and the surface being tested,

and the contrast becomes less the higher the reflectance of the surface undergoing test.

While it has been proposed to use a semi-transparent film on the reference surface of a test glass in order to increase the contrastbetween the bands, it is an' object ,of the present inven, tion to improve upon such-test glasses by providing a more rugged and durable nlm which also further increasesA the contrast between the bands.

Other objects and advantages of the present invention will appear from the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a top plan view of the test glass of the present invention.-

Fig.2 is a vertical section of the test glass shown on a workpiece. v

Fig. 3 is apsectional view. partly in elevation,

of one form of apparatus which can be used for y reference surface ofthe test glass.

Fig. 4' illustrates a test glass of the inuse.

Fig. 5 is a view similar to Fig. 4 but showinsr the interference bands ldeveloped -in usev by the test glass of the present invention.

prior art Fig. 6 is a vertical section of a modified form of the test glass of the present invention.

The device of the present invention, referring now to the drawing, comprises atransparent evaporation` process. An apparatus which may be used for forming the film is shown in Fig. 3 and comprises ametallic base plate I2 supporting an evacuable container such as the bell jar I3. A vacuum tight seal is formed between the lower edge of the bell jar [3 and the base plate I2 by any suitable sealing composition I4. The interior of the bell jar I3 is connected. by some conventional means such as the conduit I5 to a high vacuum pump, shown here diagrammatically at IB. The body III is mounted in a suitable support I'I, held above the base plate I2 by stanchions I8, so that the reference surface faces a filament I9 supported between current conducting posts 2I passed through suitable insulating bushings 22, mounted in apertures in the base plate I2. The filament I9 is connected to a suitable source of current, not shown in the drawing. through the posts 2| and lead-in conductors. 23.

filament I9 after which the seal I I is completed,

and the bell Jar I3 partially evacuated by means of the pump I 6. The surface to be coated is then subjected to a glow discharge created by a high voltage discharge between the electrode 20,

insulatedly mounted in the base plate I2, and the base plate itself. The bell jar is then furtherv evacuated and: the filament I9 is energized to heat the U shaped strip of the metal to the evap oration temperature thereof whereby a distilled 'filmA of the metal is formed on the underneath surface of the member I0.

The film is formed by. evaporating the alloy, sold under the trade name InconeL which comprises substantially 80% nickel, 14% chromium, and 6% iron. A film of Inconel produced by a. high vacuum thermal evaporation process is quite rugged, in that, it is resistant to l frlctional wear'and isv highly resistant to atmospheric corrosion. As the lm' is rugged and resistant to wear, ,the reference surface of the test glass can be placed in direct contact with the surface 24 to be tested, as'shown in Figs. 2 and 5.

Furthermore, it is substantially neutral in color whenl viewed by reflected light so that a test glass of the present invention can be effectively used in ordinary white light as well as monochromatic light.

The metal film I I increases the reflective power of the reference surface of the test glass, so that it is more comparable with that of the surface or surfaces which are to be tested with the glass.

Although the thickness of the illm is not abso-v lutely critical; it has been found that a film of a thickness suillcient to reduce rtransmission ap' proximately 50% gives the-best .contrast when used with polished metal or other highly redecting surfaces. A film of a thickness of approximately 0.004 micron will reduce transmission substantially 50%, although lms having thicknesses as great as 0.02 micron will greatly enhance the contrast.

The alteration in contrast brought about by the semi-transparent illm on the reference surface of thetest glass is due to the increased number of multiply reflected rays in the air space between the reference surface of the. glass and the surface tested. The increased number of multiply re.

flected rays greatly enhances the contrast between the interference bands. This is illustrated by Figs. 4 and 5 of the drawing, Fig, 4 illustrating the ,contrast to be expected where the reference surface of the test glass 25 is not filmed. while F18. 5 illustrates the interference bands set up by the test glass I0 of the present invention.

The changes in the character of the interference fringes with the lm -of Inconel are unexpected, in that, the changes are different than those obtained with films of other metals such y reference surface by the evaporation process de-I The narrow bright lines produced by the Inconel iilm on the test glass are such as can be face of highly polished cylindric`a1 bearing surfaces.

If the film I I should become worn through con-v tinued use, the worn film can easily be removed by hydrochloric acid, and a new ilm laid down on the scribed heretofore.

It is to vbe understood that while a preferred embodiment of the present invention has been described and illustrated herein, the invention is 2 5 not to be limited thereby, but is susceptible of changes in form and detail within the scope of' the appended claim,

I claim:

A device for indicating the deviation of a highly so polished metal surface from a predetermined as aluminum. With the lattermetal, the film produces'narrow dark lines on a bright background when used in reflected monochromatic light, while an Inconel film produces narrow bright lines on a black or gray background when viewed by reected monochromatic light. This l difference in `behavior can probably be explained by the relatively low renecting power of the glass- Inconel interface, together with the higher absorbing power of Inconel as compared with aluminum or sliver.

standard surface comprising a transparent member having an optically correct reference 'surface forming the standard surface against which is positioned the surface to be tested; and a semitransparent filmof an alloy comprising approximately 80% nickel,l l4%chrom.ium,'and 6% iron, adhering to said reference surface, said lm havinga uniform thickness of .004-.02 microns and substantially reducinglight transmission through 40 said transparent member.

` ARTHUR F. TURNER.. 

